1. Field of the Invention
This invention relates to a method of recording information. In particular, the present invention relates to recording digital signals of two values on an optical magnetic recording medium provided with a first magnetic layer and a second magnetic layer, wherein the second magnetic layer has a higher Curie point and lower coercive force at room temperature than the first magnetic layer.
2. Related Background Art
Research and development of an optical memory element which uses a laser beam has quickly realized optical media with high density and large memory capacity. However, it is optical magnetic recording medium which is advisable for use as a rewritable optical memory element.
When information recorded on an optical magnetic recording medium is to be rewritten, the information must be erased before the new information is recorded since overwriting cannot be attained. For example, when information on a predetermined track of an optical magnetic disc is to be rewritten, the information on the track is erased in one revolution of the disk and then new information is written in the following revolution. As a result, re-recording speeds are low.
In order to solve the above problem, prior art apparatus provide separate record/reproduce head and erase heads. Information can also be recorded by an application of a magnetic field while a laser beam is modulated. However, such an apparatus is costly, quite large and often cannot attain high speed modulation.
On the other hand, an overwritable optical magnetic recording medium without the above drawbacks which enables overwriting similarly as magnetic recording media using a recording device constructed according to the prior art with a magnetic field generating means and a recording method by use thereof, is proposed in Japanese Laid-Open Patent Application No. 62-175948 (corresponding to West Germany Offenlegungsschrift 3,619,618) and commonly-assigned Japanese Laid-Open Patent Application Nos. 63-153752 and 63-133338 (both corresponding to U.S. patent application Ser. No. 475,941, filed Jan. 30, 1990). Such recording methods are illustrated at FIGS. 1A and 1B.
FIG. 1A shows the magnetized state of an optical magnetic recording medium and FIG. 1B shows the power of the laser beam irradiated on the medium, wherein reference numeral 1 denotes a first magnetic layer (recording layer) and reference numeral 2 denotes a second magnetic layer (initialization layer) having a higher Curie point and lower coercive force at room temperature than the first magnetic layer 1. As shown in FIG. 1B the laser beam with intensity modulated to "high" or "low" (corresponding to the recording signal "1" or "0", respectively) is irradiated to record information. When the medium after recording is initialized by the initialization magnetic field 6, it attains the magnetized state shown in FIG. 1A.
In FIG. 1A, for example, as the recording layer and the initialization layer, rare earth-iron group amorphous alloy thin films having the same compositions relative to the compensation composition are used. Reference numeral 4 denotes a Bloch magnetic wall formed on the side faces of the cylindrical magnetic domain 3 recorded on the recording layer 1, and reference numeral 5 denotes an interface magnetic wall formed at the boundary between the cylindrical magnetic domain 3 and the initialization layer 2. As used herein, the term "magnetic wall" refers to a transition region where the magnetic moment changes its direction gradually. The magnetic wall therefore accumulates energy of a certain magnitude and constantly tends to make its area smaller.
In the above example, since both the Bloch and interface magnetic walls tend to make their areas smaller (that is, the region of stable magnetization domain tends to be enlarged), both act to collapse the recording magnetization domain. The initialization magnetic field 6 also acts to make the area of the Bloch magnetic wall smaller. Therefore, the recording magnetic domain becomes unstable.